Marking instrument



FIG. 1 is a left side elevational view of a marking instrument, in an exploded condition, showing our new design;

FIG. 2 is a front elevational view in an exploded condition;

FIG. 3 is a bottom plan view;

FIG. 4 is a right side elevational view; and

FIG. 5 is a top plan view thereof. 

The ornamental design for a marking instrument, as shown and described. 